Extraction of polyphenols from grape skins and defatted grape seeds using subcritical water: Experiments and modeling

• Polyphenols extracted from grape skins and defatted seeds with subcritical water SW.
• Operative conditions: 10 MPa, 80–120 °C, 2–5 mL/min, 2 h.
• Polyphenols yield: higher at high temperature and low solvent flow rate.
• Polyphenols yields: higher for grape seeds (124 ± 1 mg/g) than for skins (77 ± 3 mg/g).
• Two-site kinetic model applies satisfactorily to SW extraction of polyphenols.

Polyphenols were extracted from grape skins and defatted grape seeds (cultivar: Pinot Nero) by using subcritical water in a semi-continuous mode. Extractions were performed at a pressure of 10 MPa, at three different temperatures (80, 100 and 120 °C) and with two water flows. For both skins and defatted seeds, total polyphenol (TP) yield significantly increased with temperature: for skins from 44.3 ± 0.4 to 77 ± 3 mg/g, while for defatted seeds from 44 ± 2 to 124 ± 1 mg/g when the temperature increased from 80 to 120 °C. Importantly, TP yield decreased with flow rate at constant temperature. The extraction kinetics was simulated by a simple model from literature. The model adjustable parameters were calculated by best fitting procedures with experimental data: the model fitted the experimental kinetics curves in a satisfactory way, providing reference values for the adjustable parameters. The present research outlines the potentialities of using subcritical water for extracting valuable polyphenols from food processing by-products, and the effect of the operating conditions. Moreover, it proves that a very simple model from literature allows the interpretation of the extraction curves on the base of mass transfer and partition coefficients.